Generally, coatings on a vehicle body such as an automobile can be formed, for example, by applying an electrodeposited coating film on a steel panel as a substrate, an intermediate coating film thereon, and a top coating film comprising a base coating filth and a clear coating film in this order. Conventionally, each coating film has been developed depending on the required properties and functions. The coating composition to form the desired coating film is prepared, and then applied. Herein, the applied coating film is baked and cured before a further application of a coating film thereon. If some kinds of coating compositions are applied in a certain order, the underlying coating film should be completely finished and smoothed before applying a further coating composition thereon. If not so, adjacent two coating layers adversely effect on each other. The rough surface of the unfinished underlying coating film protrudes on the further applied overlying coating film, and then the resulting multilayer coating film has a deteriorated exterior appearance.
In recent years, in the field of coatings on vehicle bodies such as automobiles, there is coming a method for producing a multilayer coating film comprising steps of applying a coating composition on an uncured coating film, and then simultaneously curing them, in order to obtain more workable efficacy and realize reduced energy to be used, which is especially demanded in recent years, such as a three coating and one baking (3C1B) procedure comprising a step of simultaneously baking and curing an intermediate coating film, a base coating film and a clear coating film, etc.
Furthermore, during such coating procedure such as a three coating and one baking (3C1B) procedure, wherein a coating composition is continuously applied on an uncured intermediate coating film, if the initially applied intermediate coating composition is sagged, the resulting coating film has a significantly deteriorated exterior appearance. Accordingly, there is a large demand on art to suppress sagging of the applied intermediate coating composition during the coating procedure in this field.
JP-A-2003-105257 (patent literature 1) discloses a waterborne intermediate coating composition comprising a carboxyl group-containing waterborne polyester resin and a melamine resin, which can provide a good hiding ability relative to the underlying coating film and a good smoothness of the resulting coating film. The patent literature 1 discloses a method for producing a multilayer coating film comprising steps of applying the waterborne intermediate coating composition to from an intermediate coating film, and baking and curing the intermediate coating film to form a cured intermediate coating film, and then, subsequently, applying a top coating composition thereon, and heating and curing it to give a multilayer coating film. Therefore, the patent literature 1 does not disclose application of the waterborne intermediate coating composition to any three coating and one baking (3C1B) procedure. Although the patent literature 1 discloses that the waterborne intermediate coating composition has been developed, in detail, in order to provide a good hiding ability relative to the underlying coating film and a good smoothness of the resulting intermediate coating film, a waterborne intermediate coating composition applicable to the 3C1B procedure is not investigated in the patent literature 1 wherein the waterborne intermediate coating composition is baked and cured after its application. Therefore, sagging of the applied waterborne intermediate coating composition during the 3C1B procedure is not improved in the patent literature 1 at all.
JP-A-2003-251264 (patent literature 2) discloses a method for producing a multilayer coating film, by a three coating and one baking (3C1B) procedure, which comprises a step of simultaneously heating and curing an intermediate coating film, a base coating film thereon, and a clear coating film thereon, which can reduce miscibility between the intermediate coating film and the base coating film, and provide a multilayer coating film having an excellent exterior appearance without yellowing. The patent literature 2 only discloses sagging of the intermediate coating film of the applied intermediate coating composition during the 3C1B procedure is suppressed by controlling viscosity of the intermediate coating film at a given shear rate.
JP-A-2003-251275 (patent literature 3) discloses a method for producing a multilayer coating film, by a three coating and one baking (3C1B) procedure, which comprises a step of simultaneously heating and curing an intermediate coating film, a base coating film thereon, and a clear coating film thereon, wherein used is a waterborne intermediate coating composition having an improved viscosity, which can provide a coating film with an excellent exterior appearance. The patent literature 3 discloses, in order to form the intermediate coating film, a waterborne intermediate coating composition comprising an acrylic emulsion resin, a melamine resin, a pigment-dispersed paste with a dispersing agent, and thickener. The patent literature 3 discloses, as the preferable thickener, a specific urethane associative thickener in order to improve viscosity of the coating composition, and therefore, which can suppress sagging of the applied coating composition during the coating procedure.
JP-A-2003-251276 (patent literature 4) discloses a method for producing a multilayer coating composition, by a three coating and one baling (3C1B) procedure, which comprises a step of simultaneously heating and curing an intermediate coating film, a base coating film thereon and a clear coating film thereon, which can provide the resulting coating film with an excellent chipping resistance, an excellent impact resistance, and an excellent exterior appearance. The patent literature 4 discloses sagging of the applied waterborne intermediate coating composition during the coating procedure can be suppressed by only adding a specific thickener or controlling thickness of the dried intermediate coating film.
WO 2007/013558 (patent literature 5) discloses an addition of a specific viscosity improving agent to a waterborne metallic color base coating composition in order to improve flip-flop (FF) property of the luster color pigment contained therein. The patent literature 5 only discloses a two coating and one baking (2C1B) procedure comprising steps of applying the waterborne metallic color base coating composition, and applying a clear coating composition thereon. The patent literature 5 does not disclose a coating procedure comprising any three coating and one baking (3C1B) procedure. The patent literature 5 does not disclose such specific viscosity improving agent is added to any intermediate coating composition.